1. Field of Invention
The present invention relation to a system for Non-contact Optical Coherence Tomography (OCT) imaging of the central and peripheral retina.
2. Related Art
Ophthalmic Optical Coherence Tomography (OCT) of the eye was originally developed by obtaining cross-sectional images of the sensory retina and retinal pigment epithelium. Recently, spectral domain OCT became available, a new technique that allowed major improvements particularly regarding image acquisition speed and image resolution. However, existing instruments do not scan the retinal periphery. The OCT scan is typically restricted to the central<40° of the retina.
OCT is presently used (in opthalmology) to evaluate only either the thickness of the central retina in macular diseases or separately the status of the optic nerve head in glaucoma patients. Furthermore obtaining the OCT pictures require dilatation of the pupil prior to taking pictures. OCT can not be performed in patients with a constricted pupil; since presently available optics do not permit it. In convention systems, the existing unbearable reflexes created by their optical elements make it impossible for the operator to simultaneously see the retina and focus on the desired area (e.g., the macula or the Optic Nerve head). In addition, the field of view is very limited; thus, important regions in the peripheral retina can not be visualized with current OCT systems and the system needs a skilled personal to handle the instrument.
Present OCT systems generally employ a Fundus Camera Design. A fundus camera is a complex optical system for imaging and illuminating the retina. Due to its location the retina must be imaged and illuminated simultaneously by using optical components common to the imaging and illumination system.
Conventional Fundus Cameras used for OCT generally include an objective lens which forms an intermediate image of the retina in front of a zoom lens, designed to accommodate for the refractive error of the patient, which relays the intermediate image to the CCD. Light travels through the objective lens in both directions making the consideration of back reflections important. On the illumination side, the objective lens images an annular ring of light onto the pupil; therefore, the need for dilation of the pupil. This ring of light then disperses to give a near uniform illumination of the interior retinal surface. The objective lens also serves a role in the imaging optics. It captures pencils of light emanating from the eye and forms an intermediate image of the retina. This intermediate image is then relayed by additional optics to a digital imaging sensor or film plane.
The objective lens also serves as the limiting factor in the field of view of the camera. FIG. 1 shows the relationship between the objective lens and the eye.
Bundles of rays leaving the periphery of the retina emerge from the emmetropic eye as a roughly collimated bundle of rays. This bundle must pass through the edge of the objective lens in order to become part of the fundus image. Bundles coming from more eccentric points on the retina cannot be captured by the objective and therefore cannot be seen in the fundus image. One method of increasing the field of view in this conventional configuration is to increase the size of the objective lens as seen in FIG. 2.
However, increasing the diameter of the objective lens causes an increase in the aberration of these lenses with size. Current practical limits taking this approach lead to a roughly 40-degree field of view seen in modern fundus cameras. An alternative method for increasing the field of view is to move the objective lens closer to the eye which has also its own limitations. The extreme case for this technique is where a portion of the objective lens actually comes in contact with the cornea. One drawback to this configuration is patient aversion due to the proximity of the lens, and increased risk of infection and corneal abrasion.